There are many grease points on a truck and trailer, and these should be serviced, at the least, as the manufacturer recommends. We lube the chassis every 15,000 miles, or as close as we can to that. We use a good synthetic grease designed for all points, front end, driveline, lube points on the brakes, and all other points including the landing gear and lift gate. On the frontend there are several points on each side, two on each kingpin and one on each tie-rod end. The moving parts of the front brakes, the slack adjusters, automatic adjust or not, and the "S" cams need greased. Some clutches have lube points, grease these sparingly. Most drivelines have grease able u-joints and slip-yokes, those that don't are called permalube drivelines and don't need regular service.
As you move under the truck, look at everything, if it looks loose, grab and shake it. If it shouldn't move and does, get it fixed. Check the transmission and differential oil levels as you go by. There are brakes on each wheel end; lube needs are the same as the steer axle brakes. Check the brake shoe thickness and look for any oil on the brake shoes that would indicate a seal leaking, any oil, get it fixed. Look for any hoses or wires rubbing or chafing, use wire ties to tie them up and away from each other. Just look at everything under the truck, rust streaks may indicate loose or cracked components.
The fifth wheel top plate uses a different type of lube but needs lubed also, packets of grease specifically designed for this purpose are available at most truck stops. The fifth wheel pivot point has grease fittings, if it is a slider, I would use penetrating oil.
The trailer needs greased also, landing gear and brakes. Any other accessories that move may need greased like the lift gate. So there you go, in and under the front, out the back, job done for another 15,000 miles.